Parser doesn’t parse BinaryExpression with comma operator as JSXExpression

[andrewbranch]

TypeScript Version: 9052804

Search Terms: JSX comma expression BinaryExpression

Filing for discoverability. Discovered while working on #25487 that we don't parse either of these correctly:

const x = <div x={p, p} />
const y = <div>{p, p}</div>

In both cases, we should parse a JSXExpression whose expression is a BinaryExpression with a CommaToken as the operator. Both Flow and Babel treat this as valid syntax (they call the expression a "SequenceExpression").

• JSX Spec
• Babel playground
• Flow playground
• TypeScript playground

[andrewbranch]

This seems to be an inconsistency between the JSX spec and implementation. The spec says that an AssignmentExpression is valid in those positions, which is exactly what we do. However, it seems they're allowing any Expression there, since that's what covers the comma operator. I don't think this is the same parser they still use, but I found some evidence showing that indeed, JSX expressions were filled with parseExpression().

[weswigham]

babel/babel#8604

[andrewbranch]

Ah, yep. Good call.